Bio-Derived and Cost-Effective Membranes with High Selectivity for Redox Flow Batteries Based on Host-Guest Chemistry.

Redox flow batteries (RFBs) stand out as a promising energy storage system to solve the grid interconnection problems of renewable energy. Membranes play a critical role in regulating the performance of RFBs, and the selectivity is commonly controlled via either size exclusion or Donnan exclusion. Membranes typically account for 40% of the stack cost of RFBs, and it is essential to develop cost-effective membranes with high selectivity to achieve widespread application. Here, a type of membrane composed of highly abundant materials derived in nature, based on a scalable fabrication process, is reported. Moreover, high selectivity is achieved attributed to the host-guest interactions between membranes and redox species, which effectively alleviate the crossover of redox-active molecules. By incorporating starch into a chitosan matrix for zinc-iodine RFBs, the highly selective recognition of starch and chitosan (host) toward triiodide (guest) builds a "wall" to block the triiodide-based active materials, meanwhile, the conducting properties of such a membrane are not compromised. The proof-of-concept battery delivers a Coulombic efficiency of 98.6% and energy efficiency of 77.4% at a current density of 80 mA cm -2 , showing the promise of such a novel and cost-effective membrane design beyond traditional selectivity chemistry.